Small cryocoolers are important tools to maintain cryogenic environment for industrial applications of superconducting magnet and cryoelectronics. In this lecture, basic understanding of thermodynamic cycles and efficiency, configuration of practical cryocoolers and several comments for users are presented.
A laser Doppler velocimeter (LDV) was applied to the measurement of thermal counterflow jet in He II, which is capable of direct measurement of the normal fluid velocity at the jet nozzle exit. The analysis of velocity measurement suggests that the normal fluid jet entrains the surrounding superfluid, which result in the zero relative velocity of two fluid even very close to the nozzle exit. It is demonstrated that the second sound Helmholtz oscillation measurement is of use as a diagnostics for superfluid turbulent state. The frequency and the decay of the oscillation were investigated for this purpose. It is also suggested that a weak superfluid turbulent state may exist in small heat input case. While, in large heat input case, the turbulent state was identified as the Gorter-Mellink type superfluid turbulent state.